Re: [PATCH] cpufreq: Avoid unnecessary locking in show() and store()
From: Rafael J. Wysocki
Date: Fri Feb 12 2016 - 08:17:41 EST
On Friday, February 12, 2016 12:01:15 PM Viresh Kumar wrote:
> On 11-02-16, 02:25, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> >
> > The show() and store() routines in the cpufreq core don't need to
> > acquire all of the locks to check if the struct freq_attr they want
> > to use really provides the callbacks they need as expected, so change
> > them to avoid doing that.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > ---
> > drivers/cpufreq/cpufreq.c | 27 +++++++++++----------------
> > 1 file changed, 11 insertions(+), 16 deletions(-)
> >
> > Index: linux-pm/drivers/cpufreq/cpufreq.c
> > ===================================================================
> > --- linux-pm.orig/drivers/cpufreq/cpufreq.c
> > +++ linux-pm/drivers/cpufreq/cpufreq.c
> > @@ -862,13 +862,11 @@ static ssize_t show(struct kobject *kobj
> > struct freq_attr *fattr = to_attr(attr);
> > ssize_t ret;
> >
> > - down_read(&policy->rwsem);
> > -
> > - if (fattr->show)
> > - ret = fattr->show(policy, buf);
> > - else
> > - ret = -EIO;
> > + if (!fattr->show)
> > + return -EIO;
> >
> > + down_read(&policy->rwsem);
> > + ret = fattr->show(policy, buf);
> > up_read(&policy->rwsem);
> >
> > return ret;
> > @@ -881,20 +879,17 @@ static ssize_t store(struct kobject *kob
> > struct freq_attr *fattr = to_attr(attr);
> > ssize_t ret = -EINVAL;
> >
> > - get_online_cpus();
> > -
> > - if (!cpu_online(policy->cpu))
> > - goto unlock;
> > + if (!fattr->store)
> > + return -EIO;
> >
> > - down_write(&policy->rwsem);
> > + get_online_cpus();
> >
> > - if (fattr->store)
> > + if (cpu_online(policy->cpu)) {
> > + down_write(&policy->rwsem);
> > ret = fattr->store(policy, buf, count);
> > - else
> > - ret = -EIO;
> > + up_write(&policy->rwsem);
> > + }
> >
> > - up_write(&policy->rwsem);
> > -unlock:
>
> I have no problems with the patch as is, but how are we going to benefit from it
> ?
>
> 'if (fattr->show/store)' is never ever going to fail, unless we have a bug here.
Well, having a check that never fails is certainly unuseful.
> So, even we may want to add a WARN_ON() for that case instead.
I can add WARN_ON()s just fine.
---
From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Subject: [PATCH] cpufreq: Avoid unnecessary locking in show() and store()
The show() and store() routines in the cpufreq core don't need to
acquire all of the locks to check if the struct freq_attr they want
to use really provides the callbacks they need as expected, so change
them to avoid doing that.
While at it, add WARN_ON()s around those checks as they are only supposed
to ever fail if there's a bug in the code.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---
drivers/cpufreq/cpufreq.c | 27 +++++++++++----------------
1 file changed, 11 insertions(+), 16 deletions(-)
Index: linux-pm/drivers/cpufreq/cpufreq.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq.c
+++ linux-pm/drivers/cpufreq/cpufreq.c
@@ -862,13 +862,11 @@ static ssize_t show(struct kobject *kobj
struct freq_attr *fattr = to_attr(attr);
ssize_t ret;
- down_read(&policy->rwsem);
-
- if (fattr->show)
- ret = fattr->show(policy, buf);
- else
- ret = -EIO;
+ if (WARN_ON(!fattr->show))
+ return -EIO;
+ down_read(&policy->rwsem);
+ ret = fattr->show(policy, buf);
up_read(&policy->rwsem);
return ret;
@@ -881,20 +879,17 @@ static ssize_t store(struct kobject *kob
struct freq_attr *fattr = to_attr(attr);
ssize_t ret = -EINVAL;
- get_online_cpus();
-
- if (!cpu_online(policy->cpu))
- goto unlock;
+ if (WARN_ON(!fattr->store))
+ return -EIO;
- down_write(&policy->rwsem);
+ get_online_cpus();
- if (fattr->store)
+ if (cpu_online(policy->cpu)) {
+ down_write(&policy->rwsem);
ret = fattr->store(policy, buf, count);
- else
- ret = -EIO;
+ up_write(&policy->rwsem);
+ }
- up_write(&policy->rwsem);
-unlock:
put_online_cpus();
return ret;